The present invention relates to a method and an apparatus for writing clock data to a storage medium.
Information for systems such as data processing systems is typically stored on storage media. Particular use is made of storage disks such as magnetic disks, opto-magnetic disks, and the like. One type of magnetic disk arrangement is a so-called xe2x80x9chead disk assemblyxe2x80x9d which is intended normally to be permanently fixed in a data processing system; the head disk assembly includes one or more magnetic disks and the associated read and write head or heads (the xe2x80x9cproductxe2x80x9d head or heads) which write data to the disk and read data from the disk. Another type of magnetic disk arrangement is of a type known as xe2x80x9cremovable mediaxe2x80x9d which normally consists of a magnetic disk medium in a protective plastics case which can be used to transfer data between data processing systems by physical transfer of the disk itself from one machine to another.
In a known method of manufacturing storage media such as hard disk drives, a head disk assembly consisting of the product head(s), the disk or disks, the motor and arm electronics, is mounted in a mastering station known as a servo-writer. The servo-writer writes a pattern of magnetic information (the xe2x80x9cservo track patternxe2x80x9d) onto the disk. The servo track pattern becomes the master reference which is used by the disk drive during normal operation in order to locate the tracks and sectors on the disk for data storage and retrieval. Clearly, the servo track pattern has to be accurately written to the disk at very well defined positions.
In order to be able to write the servo pattern onto the correct positions on the disk during manufacture of the disk, in one commonly used process, a (usually temporary) xe2x80x9cclockxe2x80x9d track is written onto the disk to serve as a timing reference during writing of the servo pattern. Conventionally, a separate clock head is used to write the clock track onto the disk and to read the clock track from the disk so that the servo tracks, which are written with the disk drive""s product head, can be phase aligned with respect to the clock track and each other. However, the use of a dedicated clock track writing head is an expensive addition to the manufacturing process and further requires that the servo tracks be written in a clean room because the clock track writing head has to be inserted into the open (unsealed) head disk assembly. Moreover, in practice the clock heads have to be replaced on a daily basis because of damage which occurs during use.
U.S. Pat. No. 5,485,322 discloses a method and system for writing a clock track on a storage medium using an internal recording head of a hard disk drive. A timing pattern is generated on the storage medium with the internal recording head and a radial positioning value used in radially positioning the internal recording head is determined. The servo pattern is written at the locations determined by the generated timing pattern and radial positioning value. In effect, in this prior art system, the clock pattern is written in an iterative manner across the disk. However, there is a problem in that this method can introduce phase differences in a clock pattern between respective tracks on the disk. As a disk will often require many thousands of clock tracks across the disk, even very small phase errors in the clock track can cumulatively become very large.
U.S. Pat. No. 5,448,429 discloses another example of a system for writing clock tracks across a disk in which a written clock signal is read to provide a reference for the writing of the clock signal to a subsequent track.
U.S. Pat. No. 5,668,679 discloses another example of a method of writing servo tracks to a disk drive in which a clock track is written followed by spiral tracks across the disk. The clock track and the spiral tracks have missing pulses or bits and are used to locate the arm of the disk drive when the servo information is written.
In any of these prior art proposals for a xe2x80x9cself-servowritingxe2x80x9d disk, as well as others not specifically mentioned herein, owing to slight deviations in the rotational speed of the hard disk, non-circularity of the disk or track, off-centre mounting of the disk, and other factors arising from operation of the electronic circuitry, the phase of the track about to be written must in practice be adjusted to ensure that the track is phase aligned with the previously written track, to ensure that precisely the correct number of clock pulses fit in one complete revolution of the disk and to ensure that these pulses are spaced as uniformly as possible and to ensure that the servo information is written in the correct place. A phase locked loop circuit, which conventionally is an analogue phase locked loop circuit, is used to lock to the clock track that has been written previously in order to ensure that the next track is phase aligned with the previously written track.
In our WO-A-98/31015, the entire disclosure of which is incorporated herein by reference, there is disclosed a xe2x80x9cself-clockingxe2x80x9d method and apparatus for writing clock data to a storage medium, such as a disk. The storage medium has tracks on which data can be stored. Clock data is written to a present track on the storage medium. The written clock data is read from that track. Clock data for a subsequent track on the storage medium is generated from the clock data read from said present track. The phase of the generated clock data for the subsequent track is compared with a reference timing signal and the phase of the generated clock data for the subsequent track is adjusted in accordance with said comparison. The comparison with a reference timing signal, which may be obtained for example from the back emf of the disk drive motor, may be carried out once per revolution of the disk. The phase adjusted clock data for the subsequent track is then written to said subsequent track. In this method and apparatus, it is again not necessary to provide a dedicated clock read/write head. As in other proposals of this type, the servo information is written to the disk using the clock track data to position the servo information very accurately. To save time, it is preferred that the servo information in a track be written alternately with the clock data in that track, i.e. for any track, a portion of clock data is written, that clock data being phase aligned with the clock data in the previous track, and then a burst of servo data is written, again appropriately phase aligned with the previous track, followed by the next portion of clock data, and so on.
The method and apparatus disclosed in WO-A-98/31015 are very useful for correcting the low frequency phase error which can occur in a self-clocking process from say the inside diameter (ID) to the outside diameter (OD) of the disk during the writing process so that the total of the phase error across the disk is reduced practically to zero. However, because the phase of the generated clock data for the subsequent track is compared with a reference timing signal to determine the phase correction required, the method and apparatus disclosed in WO-A-98/31015 are not able to correct the high frequency phase errors which can occur between clock tracks within a revolution of the disk.
According to a first aspect of the present invention, there is provided a method of writing clock data to a storage medium whilst the storage medium is rotating, the method comprising the steps of:
(A) reading a burst of clock data from a present track;
(B) generating a burst of clock data for a subsequent track on the storage medium in accordance with the burst of clock data read from said present track;
(C) adjusting the phase of the generated burst of clock data;
(D) writing said phase adjusted burst of clock data to said subsequent track; and,
(E) repeating steps (A) to (D) for plural bursts of clock data around a revolution of the storage medium thereby to write clock data to said subsequent track.
The present invention allows the bursts of clock data to be properly aligned in phase with each other, with a minimal or practically zero track-to-track phase error. This can be achieved without affecting the operation of the phase locked loop typically used in the apparatus and can be used in conjunction with phase adjustment already acting to minimise phase errors around a track on the disk or across the disk, for example from the ID to the OD. The method and apparatus therefore allows for highly accurate writing of clock data to a storage medium using for example the internal read/write elements associated with the storage medium, thereby facilitating the writing of clock and servo data to a storage medium outside of a clean room.
Preferably, the method comprises the step of measuring the phase error introduced on reading a burst of clock data from the present track, and step (C) includes the step of adjusting the phase of the generated burst of clock data on the basis of the phase error introduced on reading the burst of clock data from the present track. Thus, the phase correction introduced by the present method is calculated in this example using a history of burst to burst errors, allowing for accurate phase adjustment.
Most preferably, the method comprises the step of measuring the phase error introduced on reading plural bursts of clock data from the present track, and step (C) includes the step of adjusting the phase of the generated burst of clock data on the basis of the phase error introduced on reading said plural bursts of clock data from the present track. Thus, an averaged history of burst to burst errors can be used in calculating the required phase adjustment, providing for more accurate phase adjustment.
Steps (A) to (E) are preferably repeated across the tracks on the storage medium, with the burst of clock data read from a certain track in a step (A) being the burst of clock data written in a step (D) for that track.
Steps (A) to (E) are preferably repeated for every track on the storage medium.
In an embodiment, the method comprises the step of, prior to step (A), writing plural consecutive bursts of clock data around a revolution of the storage medium to provide a substantially contiguous clock track around a revolution of the storage medium. This provides an initial reference and source of an initial clock track for the subsequent copying of clock data across the storage medium.
According to a second aspect of the present invention, there is provided apparatus for writing clock data to a storage medium whilst the storage medium is rotating, the apparatus comprising:
a data reader for reading a burst of clock data from a present track;
a pattern generator for generating a burst of clock data for a subsequent track on the storage medium in accordance with the burst of clock data read from said present track;
a phase error adjuster for adjusting the phase of the generated burst of clock data; and,
a data writer for writing said phase adjusted burst of clock data to said subsequent track;
the apparatus being adapted to read a burst of clock data from a present track, generate a burst of clock data for a subsequent track on the storage medium in accordance with the burst of clock data read from said present track, adjust the phase of the generated burst of clock data and write the phase adjusted burst of clock data to said subsequent track for plural bursts of clock data around a revolution of the storage medium thereby to write clock data to said subsequent track.
The phase error adjuster is preferably arranged to adjust the phase of the generated burst of clock data on the basis of the phase error introduced on reading the burst of clock data from the present track.
The phase error adjuster is most preferably arranged to adjust the phase of the generated burst of clock data on the basis of the phase error introduced on reading plural bursts of clock data from the present track.